1. Field of the Invention
The present invention relates to an autofocusing optical system of a camera module, more particularly, which adopts a liquid lens with a curvature radius changing in response to a voltage applied, to achieve a smaller size and high-definition.
2. Description of the Related Art
In general, a camera is constructed of a plurality of lenses, and moves the lenses to change relative distances thereof, thereby adjusting an optical focal length. Recently, a mobile telecommunication terminal has been installed with a camera, which thus enables photographing of still and live images. Also, the camera is increasingly improved in its capability to realize high definition.
However, a conventional camera module installed in the mobile telecommunication terminal adopts a fixed focus system. This renders a focus hardly adjustable at a specific distance, thus hampering definition of an image. Therefore, the camera module of a mega pixel or more should essentially possess a focusing function.
To this end, a need has arisen to apply a camera module with autofocusing, close-up and optical zooming mechanisms to a mobile phone. However, such a conventional camera module is hard to mount on a small-sized mobile phone.
That is, in the conventional module, a relative distance between an image sensor and a lens is altered and a DC motor is employed as a driving source for focusing and/or zooming. Here, a plurality of deceleration gears are connected to each other to change a relative distance between the lenses. Thus decline in response rate and variation in rotational speed may hinder a precise control of location necessary to perform focusing accurately. Besides, the conventional camera module, which is large in size and complex in design, hardly performs autofocusing in an extremely limited space of a compact optical system such as a mobile phone.
What is more, the lenses adopted in plurality for high-definition increase manufacturing costs, and power consumption owing to mechanical operation required.
In an attempt to solve these problems, a variable focal lens has been employed to achieve autofocusing.
FIGS. 1(a) to 1(c) are schematic cross-sectional views illustrating a variable focal lens proposed in PCT Pub. No. WO 03/069380.
As shown in FIGS. 1(a) to 1(c), the variable focal lens includes a substantially cylindrical fluid chamber 65, a fluid contact layer 70, a first electrode 62 and a second electrode 72. The fluid chamber 65 has a cylinder wall, and houses first and second fluids A and B in contact over a meniscus 74, which are non-miscible and differ in refractivity. The fluid contact layer 70 is arranged on the inside of the cylinder wall. The first electrode 62 is separated from the first and second fluids A and B by the fluid contact layer 70. The second electrode 72 acts on the second fluid B.
Here, the first electrode 62 is substantially cylindrical, coated by an insulating layer 68 and made of a metallic material. The second electrode 72 is arranged at one end of the fluid chamber 65.
Moreover, a transparent front element 64 and a transparent rear element 66 are disposed to seal the fluid chamber 65 for containing the fluids.
The variable focal lens structured as above operates as follows.
When no voltage is applied between the first electrode 62 and the second electrode 72, the fluid contact layer 70 has a higher wettability with respect to the first fluid A than the second fluid B.
Due to electrowetting, the wettability by the second fluid B varies under the application of a voltage between the first and second electrodes 62 and 72, which tends to change the contact angle Q1, Q2 and Q3 of the meniscus, as shown in FIGS. 1(a) to FIG. 1(c).
In consequence, the shape of the meniscus is variable depending on the applied voltage.
That is, as shown in FIGS. 1(a) to 1(c), in accordance with the magnitude of the voltage applied, the contact angle between the meniscus 74 and the fluid contact layer 70 measured in the first fluid B varies into 1400, 1000, and 600, respectively.
Here, FIG. 1(a) illustrates a lens configuration with a high negative refractive power, FIG. 1(b) illustrates a lens configuration with a low negative refractive power, and FIG. 1(c) illustrates a lens configuration with a positive refractive power.
As described above, the variable focal lens using fluids (hereinafter “liquid lens”) is more advantageous for miniaturization compared with the lens for adjusting a focus through mechanical operation.
However, the liquid lens, if adopted alone, demonstrates definition of merely about 300 thousand pixels without assuring high definition. Thus the liquid lens finds a limited application in the current meca pixel camera.
An autofocusing optical system using such a liquid lens is disclosed in Japanese Laid-open Patent No. 2005-84387.
But in the autofocusing optical system proposed in the document, a liquid lens is disposed in front of a typical fixed focus optical system to achieve autofocsing. This however prolongs a total length of the optical system equal to the dimension of the liquid lens along the optical axis, which is disposed in front of the optical system as just described.
Furthermore, in the optical system of the above document, the liquid lens is disposed in a first lens group, and thus rendered vulnerable to drop impact. That is, since a mobile telecommunication terminal is often dropped off inadvertently, a critical factor lies in how reliably the terminal and the camera module installed therein withstand drop impact. However, in the Japanese patent, the liquid lens is disposed in the first lens group, risking great damage from external impact.
Notably, the liquid lens placed in the first lens group increases a refractive power, thus susceptible to tolerance.